Explain how to call string method (#2302)

exercises/concept/log-levels/.docs/introduction.md

@@ -8,7 +8,21 @@ A `string` in C# is an object that represents immutable text as a sequence of Un
 string fruit = "Apple";
 ```
 
-Strings are manipulated by calling the string's methods. Once a string has been constructed, its value can never change. Any methods that appear to modify a string will actually return a new string.
+### Methods
+
+Strings are manipulated by calling the string's [built-in methods][docs-string-methods]. 
+For example, to remove the whitespace from a string, you can use the [Trim method][tutorial-docs.microsoft.com-trim-white-space] like this:
+
+```csharp
+string name = " Jane "
+name.Trim()
+// => "Jane"
+```
+
+Once a string has been constructed, its value can never change. 
+Any methods that appear to modify a string will actually return a new string. 
+
+### Concatenation
 
 Multiple strings can be concatenated (added) together. The simplest way to achieve this is by using the `+` operator.
 
@@ -18,10 +32,15 @@ string name = "Jane";
 // => "Hello Jane!"
 ```
 
+### Formatting
+
 For any string formatting more complex than simple concatenation, string interpolation is preferred. To use interpolation in a string, prefix it with the dollar (`$`) symbol. Then you can use curly braces (`{}`) to access any variables inside your string.
 
 ```csharp
 string name = "Jane";
 $"Hello {name}!";
 // => "Hello Jane!"
 ```
+
+[docs-string-methods]: https://docs.microsoft.com/en-us/dotnet/api/system.string
+[tutorial-docs.microsoft.com-trim-white-space]: https://docs.microsoft.com/en-us/dotnet/csharp/how-to/modify-string-contents#trim-white-space